Combined spark plugs and oscillatory circuit



J. F. WHY

Aug. 16, 1966 COMBINED SPARK PLUGS AND OSCILLATORY CIRCUIT FiledDec. 5. 1963 2 Sheets-Sheet 1 OJ A MN. 3 r m JU d 2 F 4 0 0. 5 l 3 4 6 0 3 Maw .3 a 4 5 0 0 ugsuvixnfiw Q Li m l l 5 I! lorne y Aug. 16, 1966 W 3,267,325

COMBINED SPARK PLUGS AND OSCILLATORY CIRCUIT Filed Dec. 5. 1963 2 Sheets-Sheet 2 lnvenlor 4 Allomev United States Patent 3,267,325 COMBINED SPARK PLUGS AND OSCILLATGRY CIRCUIT John Frederick Why, Dunstable, England, assignor to General Motors Corporation, Detroit, Mich a corporation of Delaware Filed Dec. 5, 1963, Ser. No. 328,338 Claims priority, application Great Britain, Dec. 6, 1962, 46,129/62 15 Claims. (Cl. 31553) This invention relates to spark plugs for internal combustion engines.

Spark plugs for internal combustion engines commonly comprise a metal shell having internal shoulders between which is clamped a lower shouldered portion of an insulator having an axial bore within which is arranged a centre electrode assembly. The centre electrode assembly has an upper portion which extends beyond the upper end of the insulator and is adapted to be connected to the ignition distributor and thus constitutes a voltage supply terminal, the latter being connected through the intermediary of a conductive seal to a lower end portion of the electrode assembly which constitutes the firing electrode or tip and extends from the lower end of the insulator and cooperates with an earth electrode secured to the lower end of the metal shell to form an ignition spark gap. The upper end of the firing tip is embedded in the conductive seal; and the upper end of the insulator is commonly provided with a plurality of annular ribs which minimize or prevent flashover between the voltage supply terminal and the shell. The lower end of the shell is threaded so that the plug can be screwed into an opening in the combustion chamber of the engine.

A spark plug according to the invention includes elements arranged to provide at least one oscillatory circuit comprising a first capacitance connected between the voltage supply terminal and earth; a second capacitance connected between the ignition spark gap electrodes; and a spark gap and inductance connected in series intermediate the sides of the two capacitances which are connected to the voltage supply terminal.

The inductance may be connected to either side of the spark gap; and the elements of the second capacitance are preferably provided by the construction and arrangement of the firing tip portion of the centre electrode, the insulator surrounding the firing tip, and the adjacent end of the spark plug shell and earth electrode. The elements of the first capacitance are preferably provided by the centre electrode assembly, the insulator, and a metallic deposit, preferably silver, on the external surface of a portion of the insulator, preferably between the annular ribs at the upper end of the insulator and the lower portion of the insulator which surrounds the firing tip portion of the centre electrode.

The inductance is conveniently in the form of a helical spring interposed between the upper and lower portions of the centre electrode within the insulator; or the inductance may be in the form of a ferrite rod with a wire coil embedded therein. Alternatively, or additionally, an inductance may be provided by the firing tip portion of the centre electrode.

The spark gap is preferably formed by a pair of metal end caps secured to opposite ends of a spacer member of ceramic or other insulating material.

The scope of the invention is defined by the appended claims; and the invention and the method by which it is to be performed are hereinafter particularly described with reference to the accompanying drawings in which:

FIGURE 1 is a vertical section through one embodiment of a spark plug according to the invention;

FIGURE 2 is a vertical section through a second embodiment of a spark plug according to the invention;

FIGURE 3 is a vertical section through a third embodiment of a spark plug according to the invention;

FIGURE 4 is a circuit diagram of the spark plug shown in FIGURE 1;

FIGURE 5 is a circuit diagram of the spark plug shown in FIGURE 2;

FIGURE 6 is a circuit diagram of the spark plug shown in FIG. 3; and

FIGURES 7 to 9 are circuit diagrams to illustrate the operation of the spark plugs described herein.

Like parts in the various embodiments shown in the drawings bear similar references.

In the embodiment of aspark plug according to the invention shown in FIGURE 1, the spark plug centre electrode assembly 1 has an upper end portion 2 constituting the voltage supply terminal and a lower end portion or firing tip 3.the lower extremity of which projects from the lower end of the plug insulator 4 and the upper extremity of which is seated on an annular shoulder 5 within the lower end portion of the insulator and is sealed in the axial bore 6 of the insulator 4 by a conductive seal 7 which conveniently comprises a fused mixture ofglass and metal particles.

Also arranged in the axial bore 6 of the ceramic insulator 4 is a spark gap element 8 comprising two metal end caps 9, 10 on opposite ends of a ceramic rod 11, one end cap 9 abutting the voltage supply terminal 2 of the centre electrode assembly 1 and the other end cap 10 being engaged by one end of a helical spring 12 the other end of which abuts a metal plug 14 which is embedded in the conductive seal 7 at the lower end portion of the axial bore 6 of the plug insulator 4.

Intermediate its ends the ceramic insulator 4 has a shouldered portion 15 of enlarged diameter which is clamped and sealed in the metal spark plug shell 16. The upper end portion of the insulator 4 is formed with a plurality of annular ribs 17 and between the lowermost rib '17 and the shouldered portion 15 of the insulator 4 the external surface of the latter is coated with a layer of silver, as indicated by the reference Ag shown on the drawing.

The lower end of the spark plug shell is formed in the customary manner with a threaded portion 18 and at its end carries an earth electrode 19 spaced from the lower end of the firing tip 3 of the centre electrode assembly 1.

In the second embodiment of a spark plug according to the invention shown in FIGURE 2, the voltage supply terminal portion 2 of the centre electrode assembly 1 and the firing tip portion 3 thereof are similar tothose in the first embodiment described above and a portion of the external surface of the plug insulator is similarly provided with a coating of silver as indicated by the reference Ag shown on the drawings. In this embodiment the centre electrode assembly 1 includes a spark gap element 28 comprising a pair of metal end caps 29, 30 secured to opposite ends of a rod 31 of ceramic material, the lower end cap 30 being embedded in the conductive seal 7 adjacent the firing tip portion 3 of the centre electrode assembly 1 and the upper end cap 249 abutting one end of a helical spring 32 the other end of which abuts one end of an inductance 3-3 comprising a rod of ferrite 34 in which is embedded a wire coil 35, the upper end of the ferrite. rod 34 being pressed by the spring 32 into abutment with the voltage supply terminal 2 of the centre electrode assembly 1.

In a third embodiment of the invention shown in FIG- URE 3 the centre electrode assembly 1 includes two spark gap elements 38, '48 respectively comprising a pair of metal end caps 3 40 and 49, 50 fixed to opposite ends of the respective ceramic rods 41, 51, one spank gap element 48 having one end thereof embedded in the conductive seal 7 adjacent the firing tip 3 of the centre electrode assembly 1 and its opposite end abutting one end of a helical spring C) 61 -the other end of which abuts one end of the second, similarly constructed, spark gap element 38, the opposite end of which is pressed by the spring 611 into abutment with the voltage supply terminal 2 of the centre electrode assembly 1.

In the operation of the three embodiments of the invention described, assuming that the spark plug is, in each case mounted on an internal combustion engine and that the voltage supply terminal 2 of the plug centre electrode assembly 1 is connected to the distributor of the ignition system of the engine, an oscillatory circuit is formed in each spark plug by the inductance element, constituted by the springs 12 or 61, or the ferrite rod and wire coil 36, the spark gap element and the capacitance elements formed by the metallic deposit on the insulator 4 and by the construction and relative dimensions of the lower end portion of the insulator and the firing tip portion 3 of the centre electrode assembly *1 and the lower end portion of the plug shell .16. By the operation of this oscillatory circuit a transient voltage of substantially higher value than that delivered by the ignition distributor is created at the ignition spark gap electrodes.

In the operation of the plug (FIGURES 1 and 4; 2 and 5) the first capacitance C is charged during the first quarter cycle of the voltage transient supplied by the coil of the ignition distributor, such transient with ignition distributor assemblies of the kind customarily used at the present time being at a frequency of approximately 2 kc./s. The impedance of the inductance L is small at this frequency and a condition is therefore created for breakdown of the spark gap S, this condition being illustrated diagrammatically in FIGURES 7 and 8 in which C and R indicates the capacitance and resistance respectively of the spark gap S.

If C R is arranged to have a high impedance with respect to C R which indicate respectively the capacitance and resistance at the firing tip or ignition spark gap at a frequency of 2 kc./s., most of the voltage developed across C will appear across spark gap 8 and break this gap down at its sparking voltage. Once the spark gap S has become conductive, due to the sparking thereacross, current flows from C by way of the spark gap S and inductance L (FIGURES 4 and 5) to charge the second capacitance If capacitance C is arranged to have a large value compared with that of capacitance C the voltage loss from C to charge C -will only be small. If no inductance is included in the circuit the capacitance C will charge to approximately the voltage on the capacitance C immediately before the spark gap S breaks down, this condition being shown diagrammatically in FIGURE 8. If, however, an inductance L is included in the circuit a transient voltage of approximately twice the value will appear across the capacitance C as shown diagrammatically in FIG- URE 9. The inductance L, as shown in the embodiments illustrated, may be placed to either side of the spark gap S (FIGURES 1, 4 and 2,

The frequency of the transient voltage is governed by the circuit constant C L and C but since C is much larger than C f is appros c 2m LC where L is in henries C is in pf.

mately 2 mc./ s. although it could be from 2 to 20 mc./s. To obtain this frequency the inductance in the oscillatory circuit should be of the order of 1 to 2 millihenrys, the minimum practical inductance value being to 200 microhenrys.

The coating of a portion of the insulator with a conductive layer of silver or other metal ensures that the value of the first capacitance C is as large as possible.

As shown in the circuit diagram of FIGURE 6 and the embodiment of the invention shown in FIGURE 3 the voltage multiplying effect in a spark plug according to the invention may be increased by arranging several oscillatory circuits in cascade. FIGURE 6 the spark gaps S S 8;, will be progressively of increasing size in order to accept the higher voltage generated at each successive stage.

In the foregoing description of the operation of spark plugs in accordance with the invention the reference letters C, L, R and S have been used, with appropriate indices, to indicate respectively the capacitances, inductances, resistances and spark gaps shown in the circuit diagrams of FIGURES 4 to 9 in order to facilitate discussion of the electrical theory involved, it being believed that the relationship between the parts shown diagrammatically in FIGURES 4 to 9 and those pictorially represented in FIGURES l to 3 will be readily apparent to those skilled in the art.

The spark gap devices referred to herein could also be replaced by non-linear voltage devices such as Zener diodes.

Spark plugs constructed in accordance with the invention can operate despite heavy deposits on the insulator which reduce the shunt resistance of the plug to a low level, owing to the steep voltage rise obtained at the plug points and the high frequency of the voltage transients.

I claim:

1. A spark plug comprising a metal shell which when the spark plug is in use provides a connection to earth, an insulator secured in said metal shell and a centre electrode assembly secured in a coaxial bore in said insulator and having at one end a voltage supply terminal and at the other end a firing tip electrode which co-operates with an earth electrode on the shell to form an ignition spark gap therebetween, and including elements to provide during use of the plug at least one oscillatory circuit, said elements comprising a first capacitance connected between the voltage supply terminal and earth, a second capaci tance connected between the ignition spark gap electrodes, and a spark gap member and an inductance connected in series intermediate the sides of the two capacitances which are connected to the voltage supply terminal.

2. A spark plug as claimed in claim 1, in which the first capacitance is constituted by the spark plug centre electrode assembly, the insulator and a metallic deposit on the external surface of a portion of said insulator.

3. A spark plug as claimed in claim 1, in which said first capacitance is constituted by said centre electrode assembly, said insulator and a metallic deposit on the external surface of a portion of the insulator between annular ribs moulded in the upper end of said insulator and the lower portion of the insulator which encloses the firing tip electrode of said centre electrode assembly.

4. A spark plug as claimed in claim 1, in which the elements of the second capacitance are provided by the construction and arrangement of the firing tip electrode of said centre electrode assembly, the insulator surrounding the firing tip, and the adjacent end of the spark plug shell and earth electrode.

5. A spark plug as claimed in claim 1, in which the inductance is a helical spring interposed between upper and lower portions of the centre electrode assembly within said insulator.

6. A spark plug as claimed in claim 1, in which said inductance is a ferrite rod with a wire coil embedded therein.

In the embodiment shown in 7. A spark plug as claimed in claim 1, in which said inductance is provided by the firing tip portion of the centre electrode assembly.

8. A spark plug as claimed in claim 1, including a first inductance in the form of a helical spring interposed between upper and lower portions of said centre electrode assembly within said insulator and a second inductance provided by the firing tip portion of the centre electrode assembly.

9. A spark plug as claimed in claim 1, in which a first inductance is in the form of a ferrite rod with a wire coil embedded therein and a second inductance is provided by the firing tip portion of said centre electrode assembly.

10. A spark plug as claimed in claim 1, in which said spark gap member comprises a pair of metal end caps secured to opposite ends of a spacer member of ceramic insulating material.

11. A spark plug comprising a metal shell which when the spark plug is in use provides a connection to earth, an insulator secured in said metal shell and a centre electrode assembly secured in a coaxial bore in said insulator and having at one end a voltage supply terminal and at the other end a firing tip electrode which co-operates with an earth electrode on the shell to form an ignition spark gap therebetween, and including elements to provide during use of the plug at least one oscillatory circuit, said elements comprising a first capacitance connected between the voltage supply terminal and earth, a second capacitance connected between the ignition spark gap electrodes, and a spark gap member and an inductance connected in series intermediate the sides of the two capacitances which are connected to the voltage supply terminal, said first capacitance being constituted by said centre electrode assembly, said insulator and a deposit of silver on the external surface of a portion of the insulator between annular ribs moulded in the upper end thereof and the lower portion of the insulator which encloses the firing tip electrode, the elements of the second capacitance being provided by the construction and arrangement of the firing tip electrode, the insulator surrounding the firing tip, and the adjacent end of the spark plug shell and earth electrode, the inductance being a helical spring interposed between upper and lower portions of said centre electrode assembly within said insulator, and said spark gap member comprising a pair of metal end caps secured to opposite ends of a spacer member of insulating material.

12. A spark plug comprising a metal shell which when the spark plug is in use provides a connection to earth, an insulator secured in said metal shell and a centre electrode assembly secured in a coaxial bore in said insulator and having at one end a voltage supply terminal and at the other end a firing tip electrode which co-operates with an earth electrode on the shell to form an ignition spark gap therebetween, and including elements to provide during use of the plug at least one oscillatory circuit, said elements comprising a first capacitance connected between the voltage supply terminal and earth, a second capacitance connected between the ignition spark gap electrodes, and a spark gap member and an inductance connected in series intermediate the sides of the two capacitances which are connected to the voltage supply terminal, said first capacitance being constituted by said centre electrode assembly, said insulator and a deposit of silver on the external surface of a portion of the insulator between annular ribs moulded in the upper end thereof and the lower portion of the insulator which encloses the firing tip electrode, the elements of the second capacitance being provided by the construction and arrangement of the firing tip electrode, the insulator surrounding the firing tip, and the adjacent end of the spark plug shell and earth electrode, the inductance being a ferrite rod with a wire coil embedded therein interposed between upper and lower portions of said centre electrode assembly, and the spark gap member being a pair of metal end caps secured to opposite ends of a spacer member of insulating material.

13. A spark plug as claimed in claim 12, including a further inductance provided by the firing tip portion of the centre electrode assembly.

14. A spark plug as claimed in claim 1, in which said inductance has a value of 1 to 2 microhenrys.

15. A spark plug as claimed in claim 1, in which the frequency of the transient voltage developed by the oscillatory circiut is between 2 mc./ s. and 20 mc./s.

References Cited by the Examiner UNITED STATES PATENTS 1,164,082 12/1915 Gallant 315-58 X 2,415,979 2/1947 Von Mertens 31553 X FOREIGN PATENTS 1,297,896 5/1962 France.

HERMAN KARL SAALBACH, Primary Examiner,

ELI LIEBERMAN, Examiner, 

1. A SPARK PLUG COMPRISING A METAL SHELL WHICH WHEN THE SPARK PLUG IS IN USE PROVIDES A CONNECTION TO EARTH, AN INSULATOR SECURED IN SAID METAL SHELL AND A CENTRE ELECTRODE ASSEMBLY SECURED IN A COAXIAL BORE IN SAID INSULATOR AND HAVING AT ONE END A VOLTAGE SUPPLY TERMINAL AND AT THE OTHER END A FIRING TIP ELECTRODE WHICH CO-OPERATES WITH AN EARTH ELECTRODE ON THE SHELL TO FORM AN IGNITION SPARK GAP THEREBETWEEN, AND INCLUDING ELEMENTS TO PROVIDE DURING USE OF THE PLUG AT LEAST ONE OSCILLATORY CIRCUIT, SAID ELEMENTS COMPRISING A FIRST CAPACITANCE CONNECTED BETWEEN THE VOLTAGE SUPPLY TERMINAL AND EARTH, A SECOND CAPACITANCE CONNECTED BETWEEN THE IGNITION SPARK GAP ELECTRODES, AND A SPARK GAP MEMBER AND AN INDUCTANCE CONNECTED IN SERIES INTERMEDIATE THE SIDES OF THE TWO CAPACITANCES WHICH ARE CONNECTED TO THE VOLTAGE SUPPLY TERMINAL. 